on their own for basic RAID configurations, or combined to create more extravagant RAID setups: RAID Level 0 – Striping Requires: 2+ drives With only a RAID 0 setup, you have no redundancy. Striping is generally used in combination with mirroring to increase I/O write performance along with redundancy. In fact, the more disks you add to a RAID 0 array, the more likely it is that you will lose your data. All it takes is one failed drive to destroy the array and your data; data is segmented and written across multiple drives. A new write on one drive can occur before an existing write on another drive can seek to a new sector. The next write will go to the next drive, etc. If your CPU is faster than your drives, this will cause a decent performance gain when data is written. (If you are wondering, your CPU is generally much quicker than your HDDs). RAID Level 1 – Mirrored Requires: 2+ drives The concept of RAID 1 couldn’t be simpler. Data from one drive is synchronized with all of the other drives in the array. In both hardware and software forms, you can generally remove all but one drive out of the array and have the exact same set of data. Operating systems can generally enjoy improved disk read performance because they can seek to either drive at any moment in time or multiple drives simultaneously. RAID Level 3,4 – Striped with Dedicated Parity Requires: 3+ drives Now we introduce the concept of parity. No data is actually mirrored in this implementation of RAID. Instead, at least two drives are striped, and a 3rd drive is added to store parity information. When data is written to the array, a simple math/logic operation is performed to create “parity”. If there is data corruption, the parity information can be used to recreate the proper data. A RAID 3 setup allows for the failure of the parity drive. Write performance is improved with striping across the other drives in the array. This implementation is not extremely popular and in most forms is not as redundant as a mirrored setup. RAID Level 5 – Striped with Distributed Parity Requires: 3+ Drives Similar to RAID 3, but in this implementation, multiple drives in the array contain parity information. One drive will take down the functioning array, but the array will rebuild if the drive is replaced. Rebuilding from parity can be quite slow and will expose your array to complete failure with the loss of an additional drive. Loss of two drives means loss of your data. RAID 5 is generally preferred to RAID 3 and is a decent choice if your limit is three drives. RAID Level 6 – Striped with Dual Distributed Parity Requires: 4+ Drives Similar to RAID 5, but loss of up to 2 drives can occur with the array continuing to function. RAID COMBINATIONS/NESTS If we take the basic building blocks of striping, mirroring, and parity, we can create funky and exciting new combos. Yay! If your chassis has room for the drives, and your RAID card allows for this, which most do, you may find the perfect fit for a redundant, high performance array if you read on. RAID Level 1+0 or 0+1 (RAID 10) Requires: 4+ Drives My favorite implementation of RAID is RAID 10. The idea is that you create two or more mirrored sets and then stripe data across the RAID 1 sets to improve performance. This practice is considered 1+0 (mirrored, then striped). 0 + 1 is less commonly used and involves mirroring striped sets. It is less redundant than the 1+0 implementation, which can allow for one drive failure in EACH of the mirrored sets, while each drive failure in 0+1 will take down the entire striped set. RAID Levels 50 and 51 Requires: 6+ drives Not supported as much by hardware RAID cards, RAID 50 and 51 include RAID 5 striped parity sets nested with mirrored or striped sets. A RAID 50 includes 2 or more RAID 5 sets as the base for a larger striped set for improved performance. RAID 51, which is more popular, includes 2 or more RAID 5 sets as the base for a larger mirrored set for improved redundancy. IMPLEMENTATION Hardware RAID is set up by installing a RAID add-on card available in PCI, PCI-x, and other bus types, or using an on-motherboard RAID controller. After your machine powers on and runs the POST, your RAID card should allow you to hit a key or key combination to enter the RAID setup utility. From here, you can generally add drives to sets, and even sets to larger sets, to create nested/combo RAID levels. Once the array is set up, it will build itself in the background, even as you install your operating system. Assuming the OS has the proper drivers to recognize your RAID card, it will see each RAID array as a single drive. Software RAID is configured in Linux or Windows by using fdisk or the disk management utility in Device Manager, respectively. This can also be accomplished during OS installation (or afterwards depending on your partitioning choices). You will create a partition with the software RAID filesystem type on one drive and then again on the other drives. Full step-by-step tutorials are available online (search Google for “software raid”). Once the arrays are initialized, they function similarly to hardware RAID in that the operating system sees only the whole array rather than each individual drive. Behind the scenes, the OS will keep the array synchronized and rebuild it after failures. Your CPU has to deal with the upkeep of the RAID array in place of a hardware RAID controller. CONCLUSION RAID controllers exist for almost any machine and hard drive technology available for servers today. From slower and cheaper 7200rpm SATA drives to faster and more expensive SCSI, SAS, or 10K RPM SATA drives, hardware or software RAID is an almost essential option to consider for web servers in a high-availability production environment. Pick the RAID level that suites your budget and redundancy requirements, but don’t be fooled into thinking it is the only technology needed to implement a solid backup system! P! Writer’s Bio: Evan Kamlet was employed by a local computer firm in 1999 and 2000 and went on to own and operate Host4Yourself Internet Services (H4Y <strong>Tech</strong>nologies LLC and formerly Host for Yourself LLC) since it was founded in 2001. He has more than a decade of experience in all aspects of the hosting industry including marketing, business operation, and technology. 54 <strong>Ping</strong>! <strong>Zine</strong> <strong>Magazine</strong>
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